1. Field of the Invention
The present invention relates to an image data processing device capable of applying a background color lightening process to background data included in structured image data for lightening a shade of a background image.
2. Description of the Background Art
Conventionally, structured image data, such as MRC (Mixed Raster Contents), JPM (JPEG2000 Multi-Layer), etc., is known as one type of compressed image data format to compress and save still image data. The structured image data represents an image with a background and a foreground, such as characters, etc., located in front of the background. The structured image data includes three data pieces, background data, foreground data, and mask data. The background data is compressed data of a background image. The foreground data is compressed data of a foreground image indicating a color and a gross shape of the foreground. The mask data is compressed data of a mask image indicating a prescribed region of the foreground image overlying the background image. For example, when a mask image included in the structured image data has the shape of the letter “A” at a center of a sheet and a foreground image included therein has a green rectangular shape at the center of the sheet, only the green rectangular shape region of the foreground image is overlaid on the background image corresponding to the shape of the letter “A”. In general, structured image data can be compressed at a high rate without excessively deforming the shape of the foreground by compressing image data of the foreground separately from that of background.
As a technique for handling the structured image data, an image data processing device described in Japanese Patent Application Publication No. 2007-235679 (JP-2007-235679-A) is known. With the technique, the image data processing device applies a background color lightening process to the background data to lighten a shade of a background image. As a result, a consumption amount of toner used as a colorant can be reduced when an image expressed by the structured image data is formed by an image forming device without degrading the legibility of text.
However, in such an image data processing device, a problem is that a toner consumption amount cannot be effectively reduced when an image including a foreground having a relatively high coverage ratio due to inclusion of a large amount of text or a large-size text.